Your only option is to manually manipulate the database tables. You'd have to insert the appropriate records from the database backup into your production database.

This shouldn't be impossible but it will require some care since it could easily hose your install if done improperly. Depending on how important the thread(s) are you could contact someone like @Slavik or another user who provides XF services and pay them to do it for you. Otherwise, you could stand up a test instance of XF and a test database and start digging through the database tables yourself.